1. Field of the Invention
The present invention relates to a wireless communication device, a wireless communication system and a wireless communication method for reducing data packets remaining in a relay node, and for equally setting end-to-end throughput for each link within a wireless ad-hoc network.
2. Description of the Related Art
A wireless ad-hoc network system allows each of plural wireless communication devices (access points or nodes) to communicate data with the other wireless communication devices within a predetermined coverage area, without a centralized control station such as a base station for a mobile phone system. Also, it allows each of the wireless communication devices to communicate data over a long distance by relaying data from a source wireless communication device to a destination wireless communication device.
FIG. 1 shows a typical wireless ad-hoc network system. Access Points AP1-AP4 are wireless communication devices each functioning as an access point. A wireless ad-hoc network is categorized into an ad-hoc based network and a mesh based network. The ad-hoc based network is composed of wireless LAN terminals only, as shown in FIG. 2. The mesh based network is composed of wireless LAN base stations and wireless LAN terminals, as shown in FIG. 3. As used herein, “a wireless ad-hoc network system” includes both the ad-hoc based network and the mesh based network. The access points AP1-AP4 in FIG. 1 correspond to the wireless LAN terminals in FIG. 2 or the wireless LAN base stations in FIG. 3. If the wireless LAN terminals in FIG. 3 include a relay function, they are included in the access points.
In FIG. 1, when one of the access points (nodes) is located so as to communicate data (transfer packets) with the neighbor access points, the access point can communicate data directly with its neighbor access points within its coverage area. That is, data can be communicated directly between AP1 and AP2, between AP2 and AP3, and between AP3 and AP4. On the other hand, each of the access points can communicate data indirectly with its non-neighbor access points via intermediate access points. That is, data can be communicated indirectly via the intermediate access points between AP1 and AP4, between AP1 and AP3, and between AP2 and AP4. It is noted that around each of the access points AP1-AP4, there may be terminals (stations) without the relay function, and the terminals may communicate via the access points AP1-AP4.
FIGS. 4(a) and (b) show examples of data transmission in accordance with IEEE 802.11, one of the wireless LAN standards. The collision avoidance for the data transmission is based on so-called virtual carrier sense. (See Non-Patent Reference 1, for example, which discloses a wireless LAN system comprising a base station and a terminal.)
In FIG. 4(a), when the access point AP1 attempts to send data to the access point AP2, the access point AP1 sends the data after a predetermined period called DIFS (Distributed Inter Frame Space) and a random Backoff time (Step S01). In response to the data, the access point AP2 sends an ACK (ACKnowledgement) packet (Step S02).
In FIG. 4(b), when the access point AP1 attempts to send data to the access point AP2, the access point AP1 sends a RTS (Request To Send) packet indicating the following data transmission, after the predetermined perid DIFS and the random Backoff time, prior to sending the data (Step S1). In response to the RTS packet, the access point AP2 returns a CTS (Clear To Send) packet which allows the data transmission (Step S2). In response to the CTS packet, the access point AP1 sends data (Step S3), and then access point AP2 returns an ACK packet after receiving the data (Step S4). This CTS/CTS mechanism is able to solve a hidden terminal problem.
According to the approach shown in FIGS. 4(a) and (b), the number of data transmissions is limited even if the access point transmits data continuously (not interrupted by another access point data transmission), because the waiting period for Backoff is required for each data transmission.
FIGS. 5(a) and (b) show examples of data transmission in accordance with IEEE 802.11e. A TXOP (Transmission Opportunity) is introduced for QoS (Quality of Service) support and improved efficiency (See Non-Patent Reference 2, for example). In FIG. 5(b), the RTS/CTS is exchanged when the hidden terminal problem mentioned in FIG. 4(b) is happening.
In FIG. 5(a), when the access point AP1 attempts to send data to the access point AP2, the access point AP1 sends a data transmission after the predetermined period DIFS and the random Backoff time (Step S011). In response to the data transmission, the access point AP2 sends the ACK packet as well. However, the access point AP1 can send the next data transmissions continuously during a predetermined duration called “TXOP Limit”, upon receiving the ACK packet from the access point AP2.
In FIG. 5(b), when the access point AP1 attempts to send data to the access point AP2, the access point AP1 sends the RTS packet to the access point AP2 (Step S11). In response to the RTS packet, the access point AP2 returns the CTS packet which allows the data transmission (Step S12), and then the access point AP1 starts sending data (Step S13) as well. However, the access point AP1 can send the next data transmissions continuously during the predetermined duration “TXOP Limit” (Step S15 and S17), upon receiving the corresponding ACK packets from the access point AP2 (Step S14, S16 and S18).
According to the approach shown in FIG. 5, the amount of data to be transmitted per unit time (Packet Transmission Rate) is increased and efficient data transmission is possible, because the waiting period for Backoff is not required, compared to the approach shown in FIG. 4.
[Non-Patent Reference 1] ANSI/IEEE std 802.11, Wireless LAN medium access control (MAC) and physical layer (PHY) specifications, 1999
[Non-Patent Reference 2] IEEE P802.11e/D9.0, August 2004
[Non-Patent Reference 3] Fusao Nuno, Ichihiko Toyoda, and Masahiro Umehira, “Performance evaluation of QoS Control Scheme that uses back pressure traffic control,” PIMRC2004, Vol. 2, pp. 830-834